Electrostatic spray guns are well known including those sold under the PRO GUN(trademark) trademarks by Graco Inc. Such guns utilize a self-contained air turbine alternator combination in conjunction with a multiplier to produce voltages of 35 kV and up.
In the instant invention, a charging electrode is introduced in or near the point of atomization similar to a standard electrostatic spray gun. As described in U.S. Pat. No. 5,647,543, the contents of which are incorporated by reference, ground electrodes are placed close enough to the high voltage electrode to create a significantly high field strength and corona region at the end of the high voltage electrode when 10 to 20 kV of charge is applied.
It has been found that the optimum position for such ground electrodes is on the side of the gun body approximately 1.5 inches from the face of the air cap and utilizing a voltage of approximately 20 kV. When the ground electrode is placed at any position adjacent to the air cap (such as shown in the aforementioned patent), transfer efficiency and charging may be high but paint can build up quickly on the ground electrodes. This is due to the fact that the atomization process creates a number of stray particles just outside the normal spray envelope and these particles will be attracted to any ground close to the air cap when charging is active. By moving the ground electrodes back along the gun body a short distance, they are far enough away from the stray particles to maintain clean operation. If the ground electrodes are moved too far back, more voltage will be required to create the necessary field strength at the charging electrode.
Such a spray gun improves transfer efficiency over non-electrostatic spray guns and yet eliminates the build-up of paint which is common to other low voltage electrostatic configurations. Such a low voltage gun can be built smaller, lighter and at a lower cost than a conventional higher voltage gun. Compared to higher voltage spraying methods, the instant invention provides a charge to the atomized particles with far less stray ions which can cause other objects to charge up to unsafe levels. Lower field strength at the operator position means less voltage sensation and less paint wrap back onto the operator compared with conventional electrostatic guns, thereby achieving better operator comfort.
Under this configuration, the ground electrode""s size and position is unobtrusive and will not interfere with the spraying operation. Construction is relatively simple because the ground path does not need to pass through the air cap or air cap ring.
These and other objects and advantages of the invention will appear more fully from the following description made in conjunction with the accompanying drawings wherein like reference characters refer to the same or similar parts throughout the several views.